Thermoplastic polymers are produced, stored, and shipped as free-flowing pellets for the subsequent extrusion and fabrication by end users. A problem encountered with storage and shipping of pellets is sticking and agglomeration, which makes processing and handling of pellets difficult. Agglomeration is particularly observed when handling pellets of thermoplastic elastomers and adhesive polymers.
Metallocene-polymerized propylene-ethylene copolymers elastomers (with majority propylene) exhibit low crystallinity, typically less than 40 J/g of propylene crystallinity. Low crystallinity promotes elasticity but renders elastomers soft, typically with a Shore A hardness of less than 100. Soft pellets have tendency to agglomerate, which prevents free flow. One method for addressing softness of elastomer pellets is to incorporate into the elastomer matrix one or more polypropylene homopolymers or copolymers that exhibit high crystallinity, i.e., greater than 40 J/g. However, such modification of the elastomer compromises elasticity, extensibility, and softness.
Polyolefin-based adhesives can be derived from propylene copolymers, primarily propylene-hexene and propylene-ethylene copolymers, with the addition of tackifiers and stabilizers. Typically, such adhesives exhibit low propylene crystallinity, i.e., less than 40 J/g, and low stiffness, i.e., a Shore A hardness of less than 100. Low crystallinity is important for desirable levels of adhesion. Presently, anti-block agents are dusted onto adhesive pellets to prevent sticking and agglomeration during storage and shipping. However, anti-block agents, which exhibit high surface activity, negatively impact levels of adhesion and tackiness observed in the adhesives.
It would be desirable to have a process for making pellets of thermoplastic polymers that exhibit low levels of sticking or agglomeration during storage and shipping. It would also be desirable to have a system for shipping pellets that was substantially free of agglomeration. It would further be desirable to have thermoplastic pellets that exhibit low levels of sticking and agglomeration during storage and shipping.